There is increasing interest in liquid hydrocarbon sources other than naturally-occurring crude oil due to increase in cost, and depletion of, naturally occurring oil deposits.
One alternative source is liquid hydrocarbons made by the Fischer-Tropsch process, invented in the 1920s, in which carbon monoxide and hydrogen (which together form synthesis gas or “syngas”) are reacted in the presence of a metal catalyst to form hydrocarbons and water. Hydrocarbons formed by the Fischer-Tropsch process may be used in a wide range of applications including use as liquid fuels (e.g. diesel and jet-fuel) and as a feedstock for forming detergents, lubricants, and olefins such as ethylene and propylene.
Industrial manufacture using the Fischer-Tropsch process has been carried out using a variety of chemical reactor equipment (see the review of Fischer-Tropsch reactors provided in Reference [1]), including slurry reactors, and fixed bed reactors. The Fischer-Tropsch process has also been demonstrated using microchannel reactors.
An exemplary slurry reactor is illustrated in FIG. 1A. Synthesis gas fed from a gas inlet is passed through a slurry containing particles of the catalyst suspended in a liquid carrier, such as heavy hydrocarbons that form liquid at the operating temperature of the reactor. The liquid mixture of hydrocarbons is known as a “wax”. This reactor is a so-called three phase reactor (gas, catalyst, wax). The catalyst and wax form a slurry that is cycled through the reactor between a slurry inlet and slurry outlet. Hydrocarbon product contained in the slurry emerging from the slurry outlet is separated from the catalyst. The catalyst is fed back into the reactor and may be partially replaced with fresh or regenerated catalyst. Any unreacted gas exiting via the gas outlet may be fed back into the reactor.
The Fischer-Tropsch process is highly exothermic and so cooling coils (usually containing boiling water) are provided within the reactor. However, the temperature difference between the cooling coils and the wax needs to be kept relatively small, otherwise the significant part of the reactor volume that is in direct contact with the cooling coils would be operated at sub-optimally low temperatures due to mixing imperfections. Consequently, a large surface area is typically required for heat transfer out of the slurry.
The amount of metal tubing required for cooling a slurry reactor makes it impractical to transport the slurry reactor across land. Scaling down slurry reactors from a typical size of about 50 to 70 meters in height to a more transportable size has been found to be problematic.
Further problems associated with slurry reactors include difficulty in separation of small catalyst particles from the hydrocarbon product; high energy consumption in driving the external and the internal circulation of the slurry; and erosion of the interior of the reactor by the slurry.
An exemplary fixed bed reactor is illustrated in FIG. 1B, in which syngas is passed through long tubes (typically about 10-12 meters in length) charged with catalyst pellets. Similarly to the slurry reactor, a large surface area for the heat exchange is needed, and this is achieved by dividing the catalyst into a large number of tubes (a so-called “multi-tubular” arrangement). The heat exchange area provided by the outer surface of tubes is about 2000 m2 per 1 t/h of produced syncrude. The average temperature difference between the coolant (boiling water) and the outer rim of the bed of catalyst pellets is only about 3° C.
Other problems with fixed bed reactors containing catalyst pellets include a high pressure drop over the tubes, difficulty in maintaining a desired temperature profile along the tube length, and high selectivity to methane. The latter is a result of intra-pellet diffusive limitations and is highly undesirable given the fact that the target of the Fischer-Tropsch process is the production of liquid hydrocarbons.
Microchannel reactors have also been investigated as candidates for Fischer-Tropsch reactors. Channels of a dimension in the millimeter range are charged with catalyst pellets or foils. Microchannel reactors provide a large surface area for heat transfer and can be scaled down more effectively than fixed bed or slurry reactors; however, they also suffer from problems of high pressure drop and elevated selectivity to methane.
A further problem common to all the reactor designs described above is that the internal components (e.g. cooling coils, catalyst tubes, or plates) that separate coolant from wax must be able to withstand high operating pressures (typically 25-32 barG). A large amount of steel is required for such high pressure ratings, increasing both the weight and the cost of the reactor.
A yet further problem with the above reactor designs is that the large amount of water produced as a by-product of the Fischer-Tropsch reaction dilutes the concentration of CO and H2 in the gas phase, and this significantly lowers the productivity of the catalyst. However, the only means for this water to be removed from the reactor is as part of the main product stream.
Many of the problems discussed above arise from the need for providing significant amounts of cooling within the reactor, something which is intrinsic to the Fischer-Tropsch reaction given its highly exothermic nature. An alternative is to combine the syngas feed stream with an inert liquid stream (e.g. one comprising low-boiling hydrocarbons), partial vaporisation of which can absorb the heat generated by the reaction. This allows the reaction to be carried out in a trickle bed reactor without any cooling mechanism. However, maintaining good mixing between the liquid and the gas may be problematic. Reference [2] describes a design that attempts to overcome this problem by dividing the reactor bed into multiple reaction sections separated by static mixers. However, this does not address any of the other problems mentioned above (e.g. the dilution effect caused by the water generated by the reaction); in fact, the introduction of an inert liquid that gradually vaporises within the catalytic bed in order to remove the heat may result in further dilution of the concentrations of the carbon monoxide and hydrogen reactants.